The purpose of this study is to determine the effects of aging and gender on cytochrome P4503A drug clearance and to evaluate verapamil, diltiazem and nifed-ipine clearance as probe of 3A4-mediated drug clearance. The use of erythromycin breath tests and dextromethorphan urinary metabolite ratios as probes of CYP3A4 to address the effects of age and gender on 3A4 activity will be investigated. A subhypothesis is to compare calcium drug clearance to other in vivo probes of CYP3A4 activity.